MRI is an imaging method for magnetically exciting nuclear spin of an object that is placed in a static magnetic field, with use of a radio frequency (RF) pulse having the Larmor frequency and reconstructing an image from nuclear magnetic resonance signals generated with the excitation. In MRI, an RE coil is used to transmit an RF pulse to an imaging region to excite nuclear magnetic resonance. The resonant frequency of the RF pulse is proportional to intensity of the static magnetic field of an MRI apparatus. For example, in the case of a static magnetic field of 1.5 tesla, the resonant frequency is 63.8 MHz.
In this frequency range, the RF pulse causes an increase in body temperature of the object. Accordingly, from the viewpoint of safety, an upper limit of the energy of the RF pulse transmitted to the object is prescribed by, for example, the International Electrotechnical Commission (IEC) standard or other standards. More specifically, energy of the RF pulse absorbed by 1 kg of living tissue is referred to as a specific absorption ratio (SAR). It is prescribed that SAR values for, for example, arbitrary 10 seconds and for 6 minutes do not exceed a first or second upper limit, respectively. The upper limit varies depending on whether the imaging region is the entire body or a partial region (such as the head).
In conventional technology, in order to satisfy the safety standards with respect to the SAR, an integrated value of the energy of an RF pulse transmitted to the object is calculated for each of preceding 1 second, 5 seconds, and 10 seconds. In any one of the following three cases, a pulse sequence is changed. A first case is that the integrated value exceeds a first predetermined value for the preceding one second. A second case is that the integrated value exceeds a second predetermined value for the preceding five seconds. A third case is that the integrated value exceeds a third predetermined value for the preceding 10 seconds. In some cases, the pulse sequence is changed by stopping operation of an RF pulse generator. However, this may cause interruption of imaging operation in the middle of the operation.
Accordingly, in the conventional technology, an SAR of the entire object and/or a partial imaging region is calculated at the time of pre-scan before imaging. If the calculated partial SAR exceeds the upper limit, an alarm is displayed and then the pulse sequence is changed so that the partial SAR does not exceed the upper limit. After it is verified that a dose to the object does not exceed the upper limit of the partial SAR, imaging is performed.
In order to calculate the SAR, there are conventional technologies which, at the time of a pre-scan before a main scan, measure an energy value (or energy control value) of an RF signal attenuated by one directional coupler with a fixed degree of coupling, the attenuated RF signal being based on an amplified RF signal from one RF power amplifier included in a transmitter. There are also conventional technologies which, at the time of the pre-scan before the main scan, measure an energy value of an RF signal attenuated by a plurality of directional couplers arranged in series on a transmission line, the attenuated RF signal being based on the amplified RF signal from the one RF power amplifier.
However, in the case of the conventional technologies involving one directional coupler with the fixed degree of coupling, the accuracy of an RF output monitor may deteriorate since MRI has a dynamic range with a wide transmission gain. While imaging of local regions, such as the limbs, requires about 1.00 to 200 [W], imaging of the entire body (such as imaging of an abdominal region) requires 10,000 to 20,000 [W] depending on the imaging sequence. The RF signal attenuated by the directional coupler is detected in a detector and is subjected to analog to digital (AD) conversion by an AD converter. Accordingly, in order to support a high power (10,000 to 20,000 [W]) signal, the degree of coupling of the directional coupler is set larger within a limit of a maximum input of the detector and the AD converter. In this case, a low-power signal (100 to 200 [W]) is excessively attenuated and its signal level becomes susceptible to noise floor and offset, which causes a problem of deteriorated accuracy in detection and A/D conversion.
In the case of the conventional technologies involving a plurality of the directional couplers, the influence of a reflective RF signal may be reduced. However, since the degree of coupling is invariable, the low-power signal (100 to 200 [W]) still suffers from the problem of deteriorated accuracy in detection and A/D conversion, as in the conventional technology involving one directional coupler.